The present invention relates to a non-woven fabric and a preparation process thereof, and more particularly relates to a non-woven fabric which is obtained by bonding or intertwining a web of filaments comprising a lactic acid-based polymer having a specific composition and having excellent dimensional stability, and a preparation process of the non-woven fabric.
2. Related Art of the Invention
Non-woven fabrics are cloth like materials prepared without weaving or knitting. More particularly, non-woven fabrics are cloth-like materials prepared from webs, which are made by bulk phase fibrous materials and not yet bonded or,intertwined, by bonding or intertwining constituent filaments of the web and obtained without weaving or knitting.
Conventionally, non-woven fabrics have widely used for garment inner lining, carpets and other materials industry. Filament materials which have been used for constituting non-woven fabrics are polyethylene, polypropylene and other polyolefins; polyethylene terephthalate (PET), polybutylene terephthalate and other aromatic polyesters; nylon-6, nylon-66, nylon-610 and other polyamides; and rayons and other cellulosics. Non-woven fabrics prepared from these polymers and preparation processes for these fabrics are disclosed, for example, in Japanese Laid-Open Patent SHO 59-88961 and 59-94660.
These filament materials, however, are almost non-degradable in the natural environment or have a very low rate of degradation. Consequently, conventional non-woven fabrics prepared from these materials remain semipermanently in the soil when buried after use. When abandoned in an ocean, these filament materials impair scenery or destruct the living environment of marine organisms. Thus, disposal of these waste materials has caused serious social problems.
Alternatively, when these materials are disposed by incineration, the global environment is destructed by evolution of toxic gases and action of these gases for accelerating corrosion of incinerators also leads to problems.
On the other hand, non-woven fabrics have been developed by using filament materials which are prepared from polymers having bioabsorbability and hydrolyzability.
For example, Japanese Laid-Open Patent SHO 63-95041 discloses a process for preparing multifilaments by melt-spinning (or melt-extruding) bio-absorbable polymers such as polyglycolic acid and a glycolic acid/lactic acid copolymer, converting the multifilaments to a random web, preparing a non-woven fabric from the random web and using the non-woven fabric as a raw material of medical pledget.
In the preparation example of said non-woven fabric, polyglycolic acid chips that one used have a reduced viscosity (.eta.sp/C) of 1.5 which is measured at 30.degree. C as a solution in a solvent mixture composed of 10 parts of phenol and 7 parts of trichlorophenol. In the disclosed process, the chips were melt-spun at 245.degree. C., stretched into filaments having 35 deniers per 12 filaments, heat-treated at 108.degree. C. for 3 hours, fabricated into a tubular knit with a cylindrical knitting machine, and needle punched after superimposing the resultant four knits to obtain a non-woven fabric having scarcely distinguishable stitches.
One of the most common processes presently used for manufacturing non-fabrics is a so-called spin bonding process. In the process, a polymer is melt-extruded to form filaments, the filaments are directly captured on a screen belt and converted to webs, and the webs are hot pressed with a heat embossing roll to obtain a non-woven fabric.
The following problems are encountered in the case of manufacturing non-woven fabrics in a large amount from polyglycolic acid by the spin bonding process. That is, polyglycolic acid filaments which are melt-extruded from an extruder are in an amorphous state having almost no crystalline portion. Sufficient crystallization cannot be progressed merely by capturing on a screen belt, converting to webs, and pressing with a hot embossing roll. Further, crystallization of the polyglycolic acid filaments gradually progresses even at room temperature when allowed to stand for a long period in an amorphous state. Consequently, non-woven fabrics prepared from polyglycolic acid filaments by the spin bonding process leads to problems such as development of shrinking and wrinkles due to crystallization of polyglycolic acid during storage or in use. Further, polyglycolic acid has high rigidity and thus non-woven fabrics prepared from polyglycolic acid does not always have high flexibility and are restricted in use.
Polylactic acid is also used for degradable polymers similar to polyglycolic acid. Non-woven fabrics prepared from polylactic acid have improved flexibility as compared with those prepared from polyglycolic acid. However, non-woven fabrics of polylactic acid also cause shrinking and wrinkles and lead to problems due to poor dimensional stability.
Degradable surgical filaments prepared from polylactic acid have been known. For example, Japanese Patent Publication SHO 41-2734 discloses an absorbable surgical filament consisting of a lactic acid polymer which has an inherent viscosity of 1.0 or more at 25.degree. C. in a 0.1 wt % benzene solution and has a monofilament shrinkage of 15% or less after immersing in water at 77.degree. C. for 5 minutes. In practice, poly(L-lactic acid) or poly(D-lactic acid) having the above inherent viscosity is melt-spun and heat-treated under tension at 60.degree..about.150.degree. for 0.5.about.5 minutes and successively cooled under tension to room temperature. The filament thus obtained is described as having an excellent tensile strength and low shrinkage and as being suitable for surgical sutures.
Poly(L-lactic acid) and poly(D-lactic acid) have high crystallinity and thus heat treatment is required in order to improve dimensional stability and tensile strength of the filaments obtained from these polymers.
On the other hand, non-woven fabrics do not require extremely high tensile strength and rather require excellent dimensional stability. When the non-woven fabrics obtained from poly(L-lactic acid) or poly(D-lactic acid) are heat-treated in order to improve dimensional stability, the filaments are not heated under one-dimensional tension as disclosed in Japanese Patent Publication SHO 41-2734, but must be heat-treated under two-dimensional uniform tension. As a result, the step becomes complex and must be unfavorably carried out in a large scale. Even if the non-woven fabrics are heat-treated under such technically complex conditions, problems of developing shrinking and wrinkles are inevitable and breakage of the non-woven fabric unfavorably takes place in some cases.
Further, poly(L-lactic acid) and poly(D-lactic acid) have high crystallinity and are thus difficult to provide molded articles with flexibility by addition of plasticizers.